Epoxy-based underfill compositions are used frequently for no-flow underfill processes. Some epoxy-based underfill compositions use anhydrides or phenols as hardeners, and also use acidic compounds such as tartaric acids and/or oxalic acids as fluxing compounds. In the case of anhydrides, alcohol is added to hydrolyze the anhydride to in situ liberate the acid in the underfill composition. Underfill compositions can also contain dielectric fillers for reduction of the coefficient of thermal expansion (CTE). Underfill compositions also use additives as elastomers for stress reduction, coupling agents for adhesion promotion, and catalysts for activating the cure.
During processing, acid interacts with interconnect metallurgy to dissolve oxides on electrical contact surfaces to provide an interconnection path for wetting between the bond pad on a substrate and an electrical bump such as on a die. One electronic device includes a flip-chip and mounting substrate, among other things. One characteristic of flip-chip technology is shear stress on the solder joints during temperature cycling of the device. This shear stress is partially a result of a difference in the CTEs of the flip-chip and the mounting substrate. Die materials such as silicon, germanium, and gallium arsenide, along with their packaging materials, may have CTEs in a range from about 3 ppm/° C. to about 6 ppm/° C. Mounting substrates are usually composites of organic-impregnated fiberglass dielectrics and metallic circuitry. These substrates may have CTEs in a range from about 15 ppm/° C. to about 25 ppm/° C. Consequently, a mismatch in the CTEs exists between the flip-chip and the mounting substrate.
Solder joints are reinforced by filling the space between the flip-chip and the mounting substrate, and around the solder joints, with the underfill composition. The two main processes that are commonly used to underfill the flip-chip include the capillary underfill process and the no-flow underfill process.
A capillary underfill process typically proceeds by first aligning the solder bumps on a flip-chip with the pads on a substrate, and the solder is reflowed to form the solder joints. After forming the interconnect, the underfill is flowed between the flip-chip and the mounting substrate. Thereafter, the underfill composition is cured. Capillary underfilling can be assisted by pumping the underfill composition between the flip-chip and the mounting substrate, or by vacuum-assisted drawing the underfill composition between the flip-chip and the mounting substrate.
The effectiveness of an underfill composition depends on its chemical, physical, and mechanical properties. Properties that make an underfill composition desirable include low CTE, low moisture uptake, high adhesion, high toughness, high glass transition (Tg) temperature, high heat distortion temperature, and others. The underfill composition includes particulate filler inorganics such as silica or the like, and metal flakes or the like. The particulate filler increases the modulus and acts as a CTE intermediary for the mismatched CTEs of the flip-chip and the mounting substrate. An example of a silica-filled composition is silica-filled, epoxy-based organics.
The no-flow underfill process is another method of underfilling a flip-chip. In a no-flow underfill process, the underfill composition is dispensed on the mounting substrate or the flip-chip, and the flip-chip and the mounting substrate are brought into contact. The solder bumps on the chip and the pads on the substrate are aligned. Next, the underfill composition is cured prior to or substantially simultaneously with reflowing the solder to create the solder joints.